I have never before heard of such a rule. It appears to be more honored in the breach than in the
observance. Even on those occasions when I have seen "Mr" or "Mrs" without the "." it appeared to
be an error. I think the use of the "." is desirable after all abbreviations.

From my dictionary: note that abbreviation for "cotangent" does not require a "."
abbreviation
n.

1 a making shorter
2 the fact or state of being made shorter
3 a shortened form of a word or phrase, as N.Y. for New York, Mr. for Mister, lb. for pound, ctn for cotangent